IF

Evolution of Plant Diversity Notes

Evolution of Plant Diversity

  • Common Ancestor: Plants and green algae (charophytes) evolved from a shared ancestor; both are multicellular and photosynthetic eukaryotes.
    • Examples:
    • Chara: A complex charophyte
    • Coleochaete: A simpler charophyte

Key Traits of Plants

  • Five key traits appear in nearly all land plants but are absent in charophytes:
    1. Alternation of Generations: Life cycle alternates between gametophyte (haploid) and sporophyte (diploid) stages.
    2. Multicellular, Dependent Embryos: The embryonic stage develops within the female gametophyte.
    3. Walled Spores in Sporangia: Spores are produced in protective structures called sporangia.
    4. Multicellular Gametangia: Structures that produce gametes are multicellular.
    5. Apical Meristems: Regions of actively dividing cells at the tips of roots and shoots for growth.
  • Additional Derived Traits:
    • Cuticle: A waxy layer that reduces water loss.
    • Stomata: Pores for gas exchange, typically on the leaves.
    • Mycorrhizae: Fungal associations that enhance nutrient uptake.

Plant Life Cycle Overview

  • Contains alternating haploid (n) and diploid (2n) generations, involving both mitosis and meiosis.
  • Gametophyte (n): Produces gametes through mitosis.
  • Sporophyte (2n): Develops from the zygote after fertilization; produces spores via meiosis.

Adaptations to Life on Land

  • Moisture Retention: Mechanisms to maintain moisture within cells.
  • Structural Support: Need for support in a nonbuoyant medium.
  • Reproduction: Adaptations for reproduction without water.
  • Anchoring: Roots anchor the plant, absorbing nutrients and water from the soil.

Comparison: Aquatic vs. Terrestrial Adaptations

  • Vascular Tissue: Present in terrestrial plants for better nutrient and water transport.
  • Roots: Essential for anchoring and moisture absorption.
  • Leaves: Increase surface area for photosynthesis.
  • Stomata: Allow gas exchange while minimizing water loss.

Key Events in Plant Evolution

  1. Origin of Land Plants (about 470 mya): Nonvascular plants (bryophytes) such as mosses, hornworts, and liverworts.
  2. Origin of Vascular Plants (about 425 mya): Seedless vascular plants like ferns with flagellated sperm.
  3. Origin of Seed Plants (about 360 mya): Development of vascular tissues with lignin providing structural support, including gymnosperms and angiosperms.

Bryophytes: Nonvascular Plants

  • Three Phyla:
    • Liverworts (Hepatophyta)
    • Mosses (Bryophyta)
    • Hornworts (Anthocerophyta)
  • Dominated by the gametophyte generation, which is larger and longer-living than the sporophyte.

Seedless Vascular Plants

  • Lycophytes and Monilophytes:
    • Examples:
    • Club Mosses (Lycophyta)
    • Ferns (Monilophyta)
  • Adaptations:
    • Lignin for increased structural support.
    • Vascular tissues (xylem and phloem) for transport.

Introduction of Seed Plants

  • Key Adaptations:
    • Seeds: Protective structures surrounding the embryo and food supply.
    • Pollen: Male gametophytes that can be dispersed without water, aiding in reproduction success.
  • Types of Seed Plants:
    • Gymnosperms: Produce seeds in cones.
    • Angiosperms: Seeds develop in protective ovaries, leading to fruit formation.

Angiosperm Diversity

  • Flower Structure:
    • Flowers contain male and female reproductive structures.
    • Serve as sites for pollination and fertilization.
  • Life Cycle: Characterized by double fertilization, where one sperm fertilizes the egg while the other combines with polar nuclei to form endosperm.
  • Fruit Development: Fruits aid in seed dispersal through various mechanisms: wind, animals, or edible fruit consumption.